1. Field of the Invention
The present invention relates generally to band-stop filter circuits, and more particularly, to an improvement of a band-stop filter circuit for use in processing a signal of a high frequency band such as a SHF (Super High Frequency) band.
2. Description of the Background Art
In general, a receiver for direct satellite broadcasting using a signal of a SHF band frequency comprises an antenna for receiving a signal of 12 GHz band transmitted from a broadcasting satellite to ground, a BS converter for amplifying a received radio wave and converting the signal into an intermediate frequency signal, and a BS tuner for performing such processings as a demodulation of the intermediate frequency signal supplied from the BS converter.
FIG. 1 is a schematic block diagram showing the structure of a BS converter in such a conventional satellite broadcasting receiving apparatus. With reference to FIG. 1, a signal of a SHF band (12 GHz band) received by an antenna 60 is amplified by a high frequency low noise amplifier 61 and thereafter is applied to an image band-stop filter circuit 62 structured by a band pass filter (BPF) and the like. The image band-stop filter circuit 62 prevents a signal of an image frequency band, out of the applied signal, from passing.
The signal having passed through the image band-stop filter circuit 62 is mixed by a mixer 63 with a local oscillation signal of 10.7 GHz supplied from a local oscillator 64 and converted into an intermediate frequency signal of 1 GHz band. The intermediate frequency signal is amplified by an intermediate frequency amplifier 65 and is supplied to a BS tuner (not shown) through an output terminal 66.
FIG. 2 is a circuit diagram showing the image bandstop filter circuit 62 shown in FIG. 1. This filter circuit 62 is provided for the following reason. If an image signal is inputted to the mixer 63 and difference between the image signal frequency f.sub.3 and the above mentioned local oscillation frequency f.sub.2 is equal to that between the frequency f.sub.1 of the desired signal to be received and the local oscillation frequency f.sub.2 as shown in a spectrum diagram of FIG. 3, the desired signal and the inputted image signal are converted into signals of the same frequency band (1 GHz). After both signals are thus converted, they can not be separated from each other any more. Accordingly, it is required to remove such image signal in advance before it is inputted to the mixer 63.
The circuit 62 is formed by microstrip lines, comprising main lines 71 and 72, an input port 73, an output port 74 and a parallel stub 70 having one end connected to the node between the main lines 71 and 72 and a termination being opened. The parallel stub 70 is designed to have an electrical length .theta.70 to become one-fourth of a wavelength of a center frequency fr of an image band to be removed at the center frequency fr.
A signal pass band f.sub.1 of satellite broadcasting using a SHF band covering Japan and the United States is about 11.7 GHz to 12.2 GHz and a local oscillator frequency f.sub.2 is about 10.7 GHz. The center frequency fr of the image band f.sub.3 to be removed by the filter circuit of FIG. 2 is therefore about 9.4 GHz. More specifically, referring to FIG. 3, the image frequency f.sub.3 is obtained as follows: ##EQU1##
Accordingly, the center frequency fr thereof becomes about 9.4 GHz. In this case, the filter circuit of FIG. 2 formed on a GaAs substrate requires line length of about 2.8 mm for the parallel stub 70.
Such a circuit as shown in FIG. 1 has been conventionally formed as a MIC (Microwave Integrated Circuit) structured by connecting such circuit devices as transistors and diodes disposed on a dielectric substrate by microstrip lines.
In recent years, however, such devices as field effect transistors (FET) have been developed which use GaAs or InP material having a mobility of electrons five or six times larger than that of Si, and various attempts have been made to make such devices and various circuits such as a matching circuit and a bias circuit monolithic. The monolithic MIC is referred to as a MMIC (Monolithic Microwave IC) which is more compact and having more excellent reliability than the conventional MIC. Thus, MMIC is applied to such a circuit for receiving satellite broadcasting of a SHF band as shown in FIG. 1. A frequency converter structured as a MMIC is disclosed, for example, in "X-Band Low-Noise GaAs Monolithic Frequency Converter", by K. Honjo et al. in IEEE Transactions on Microwave Theory and Techniques Vol. MTT-33, No. 11, November 1985 pp.1231-1235 and U.S. Ser. No. 533,576, filed Jun. 5, 1990 (U.S. Pat. No. 5,065,117) by the same assignee of the present invention.
Since in a MMIC, a distributed constant circuit is formed on a semiconductor substrate of GaAs or the like, it is very difficult to form such a long line in the order of millimeter as the above-described parallel stub 70 in terms of a chip size. It is therefore difficult to form the image band-stop filter circuit shown in FIG. 2 as a MMIC.
The broken line of the graph of FIG. 4 shows a calculation result of a signal transmission characteristic (frequency characteristic) of such a filter circuit as shown in FIG. 2 formed on a GaAs substrate with a parallel stub set to have a line length of 2.8 mm as described above and the respective characteristic admittances Y70, Y71, Y72 of the parallel stub 70 and the main lines 71 and 72 set to be 0.02 mho. In FIG. 4, the abscissa shows a signal frequency (GHz) and the ordinate shows a ratio of output/input of the filter circuit or transmission loss.vertline.S.sub.21 .vertline..sup.2 (dB). The calculation result shown in FIG. 4 is obtained on the assumption that the input and output ports 73 and 74 are the terminals terminating at 50.OMEGA..
As is clear from the calculation result of FIG. 4, the filter circuit structured on the above-described conditions does not necessarily obtain a desired filter characteristic because a signal attenuation amount of a band to be removed (9.2 GHz-9.7 GHz) is relatively as small as about -20 dB, while a transmission loss in a signal pass band (11.7 GHz-12.2 GHz) is relatively as large as about 4 dB.
As described in the foregoing, forming the filter circuit shown in FIG. 2 as a MMIC on a GaAs substrate requires a reduction of a circuit area while obtaining an excellent characteristic as a band-stop filter.
Designing a filter circuit generally involves the following problems:
(1) A filter circuit implemented as a distributed constant circuit leads to an increase in the size of the circuit itself.
(2) A filter circuit implemented as a concentrated constant circuit limits an allowable inductance value due to the effect of a self-resonant frequency of a spiral inductor.
These problems make it difficult to obtain a filter circuit occupying reduced area and suitably made into a MMIC, and having a desired filter characteristic.